The invention generally relates to magnetic resonance imaging (MRI), and more particularly, to radio frequency (RF) surface coils used for MRI.
Generally, MRI is a well-known imaging technique. A conventional MRI device establishes a homogenous magnetic field, for example, along an axis of a person's body that is to undergo MRI. This homogeneous magnetic field conditions the interior of the person's body for imaging by aligning a majority of the nuclear spins of nuclei (in atoms and molecules forming the body tissue) along the axis of the magnetic field. If the orientation of the nuclear spin is perturbed out of alignment with the magnetic field, the nuclei attempt to realign their nuclear spins with an axis of the magnetic field. Perturbation of the orientation of nuclear spins may be caused by application of RF magnetic field pulses. During the realignment process, the nuclei process about the axis of the magnetic field and emit electromagnetic signals that may be detected by one or more surface coils placed on or about the person.
Imaging time is dependent upon the desired signal-to-noise ratio (SNR) and the speed with which the MRI device can fill the k-space matrix, which is transformed to create an image. In conventional MRI, the k-space matrix is filled one line at a time. Although many improvements and variants have been made in this general area, the speed with which the k-space matrix may be filled is limited. To overcome these inherent limits, several techniques have been developed to simultaneously acquire multiple lines of data for each application of a magnetic field gradient. These techniques, which may collectively be characterized as “parallel imaging techniques”, use spatial information from arrays of RF detector coils to substitute for the spatial encoding which would otherwise have to be obtained in a sequential fashion using field gradients and RF pulses.
Two such parallel imaging techniques that have recently been developed and applied to in vivo MRI are SENSE (SENSitivity Encoding) and SMASH (SIMultaneous Acquisition of Spatial Harmonics). Both techniques include the use of a plurality of separate receiving elements operated in parallel, with each element having a different or spatially translated sensitivity profile. Combination of the respective spin resonance signals detected enables a reduction of the acquisition time required for an image (in comparison with conventional Fourier image reconstruction) by a factor which in the most favorable case may equal the number of the receiving members used (see Pruessmann et al., Magnetic Resonance in Medicine, Vol. 42, pp. 952-962, 1999).
A drawback of the SENSE technique, for example, results when the component coil sensitivities are either insufficiently well characterized or insufficiently distinct from one another. These instabilities may manifest as localized artifacts in the reconstructed image, or may result in degraded SNR. Accordingly, it is desirable to implement RF coil arrays in MRI systems that (among other aspects) provide increased SNR with or without the use of parallel imaging techniques such as SENSE.
Additionally, image artifacts are also attributable to the mutual couplings between coils in a cluster of closely situated surface coils, which have been separately tuned and matched. The mutual couplings between the coils generate coupled modes, which cause splitting in the coils' resonant spectrum. Consequently, the coils become detuned and mismatched, causing reductions in the SNR. To sustain the SNR of the coils and avoid image artifacts caused by coil coupling, some electrical decoupling mechanisms are needed to collapse the multiple coupled modes into a single degenerate mode that resonates at the MRI frequency.
More recently, parallel imaging techniques have been further developed to exploit multiple receive channels, for example, 8, 16 or 32 channels receiving signals from 8, 16 or 32 receiver coils respectively. In a typical multiple coil array arrangement, several adjacent coils are provided for receiving signals during imaging. However, there are a number of design challenges in providing the capability of multiple receive channels and multiple coils. For example, the size of coils needed to support a 32-channel MRI system must be sufficiently small to fit within a typical 40 cm field of view of a conventional MRI system, or a smaller field of view for some applications. Additionally, the coil size and corresponding arrangement within a coil array will present inherent inductive coupling and sensitivity issues which both can negatively impact the quality (Q) and loading factors of the coils, thereby limiting overall SNR performance of the coils and MRI system during imaging. To address the size and sensitivity issues described above certain coil configurations, for example spiral or multiple-looped coils, have been used in coil arrays designed for magnetic resonance operating below 30 MHz. There is now an increased need for additional receive channels and MRI system capability at higher frequencies, especially but not limited to 64 MHz and 128 MHz corresponding to 1.5 T and 3 T systems, respectively. However, spiral or multiple-looped coils used in MRI systems operating at or above 30 MHz may negatively impact the overall performance of the coils and MRI system during imaging.
The loading factor is the ratio of unloaded Q to loaded Q (when the coil is loaded by being placed on the subject), where the quality factor Q is a measure of the coil resonance frequency divided by the band-width of the coil resonance, as is known to those skilled in the art. The loading factor serves as a measure of the ratio of total resistive losses arising from the coil and the imaging subject divided by the losses from the coil alone. High loading factors generally mean that most of the noise is coming from the subject, not the coil, and in the absence of strong E-field coupling between the coil and the sample, are generally interpreted as a sign of good coil performance.
What is needed is a highly coupled RF coil assembly having high Q and high loading factor MRI coils for use in a multi-channel MRI system operating at higher frequencies.